[parpfish]

I think this the main reason -- it really just comes down to the number of unique pieces of content a user will stream in a session.

For video a session is generally one movie or a bunch of episodes of the same TV show, so you can sit in one service for a long time.

But for music, you'd either spend lot of time hopping between services OR you are forced into making compromises (e.g., do you listen to the classic rock radio station on the service that has rights to rolling stones, or the app that has rights to led zeppelin)?